Many polypeptides are known which are the expression product of a single gene. A number of these polypeptides are originally synthesized as a single polypeptide chain, but contain multiple, independently folded domains, which are subject to limited proteolysis (or proteolytic cleavage(s)) in vivo that may result in separation of domains due to dissociation of the cleavage products. Proteolysis resulting in the separation of domains has been shown to alter the stability and/or enzymatic or functional activities of a variety of these proteins. Examples of these proteins include plasma proteins, such as those involved in blood coagulation.
As known in the art, blood clotting begins when platelets adhere to the wall of an injured blood vessel at a lesion site. Subsequently, in a cascade of enzymatically regulated reactions, soluble fibrinogen molecules are converted by the enzyme thrombin to insoluble strands of fibrin that hold the platelets together in a thrombus. At each step in the cascade, a protease precursor is converted to a protease that cleaves the next protein precursor in the series. Cofactors are required at most of the steps. In its active form, the protein factor VIII is a cofactor that is required for the activation of factor X by the protease, activated factor IX.
Factor VIII can be activated to factor VIIIa (where “a” indicates “activated”) proteolytically by thrombin or factor Xa. In combination with calcium and phospholipid, factor VIIIa makes factor IXa a more efficient activator of factor X by a mechanism which is not fully understood.
People deficient in factor VIII or having antibodies against factor VIII who are not treated with factor VIII suffer uncontrolled internal bleeding that may cause a range of serious symptoms, from inflammatory reactions in joints to early death. Severe hemophiliacs, who number about 10,000 in the United States, can be treated with infusion of factor VIII, which will restore the blood's normal clotting ability if administered with sufficient frequency and concentration.
Several preparations of human plasma-derived or recombinant factor VIII of varying degrees of purity are available commercially for the treatment of hemophilia A. These include a partially-purified factor VIII derived from the pooled blood of many donors that is heat- and detergent-treated for viruses but contains a significant level of antigenic proteins; a monoclonal antibody-purified factor VIII that has lower levels of antigenic impurities and viral contamination; and recombinant human factor VIII.
Hemophiliacs require daily replacement of factor VIII to prevent the deforming hemophilic arthropathy that occurs after many years of recurrent hemorrhages into the joints. However, supplies of factor VIII concentrates have never been plentiful enough for treating hemophiliacs adequately because of problems in commercial production and therapeutic use. For example, the commonly used plasma-derived factor VIII is difficult to isolate and purify, is immunogenic, and requires treatment to remove the risk of infectivity from AIDS and hepatitis viruses. Porcine factor VIII may also present an alternative, however a limitation of porcine factor VIII is the development of inhibitory antibodies to it after one or more infusions.
Activated factor VIII (FVIIIa) is thermodynamically unstable under physiological conditions due to the tendency of the A2 domain to dissociate from the rest of the complex. In other words, activated FVIII spontaneously becomes inactive. If this dissociation could be prevented in pharmacological preparations of FVIII or FVIIIa, administration that is less frequent and/or of lower concentration, could be realized. This could result in a number of benefits such as cost savings, decreased use of medical personnel, and improved lifestyle for hemophiliacs.
Another plasma protein besides factor VIII is prothrombin. As part of the coagulation cascade, prothrombin is converted to thrombin by the action of the prothrombinase complex (FXa, FVa, and Ca2+). In human prothrombin, this conversion involves cleavages at Arg271 and Arg284, between the F2 domain and the thrombin A chain, and at Arg320, between the A and B chains (human numbering system). In vivo, prothrombinase first cleaves prothrombin at Arg320, producing meizothrombin. Free meizothrombin is an unstable intermediate, and autolysis at the Arg155-Ser156 bond rapidly removes the F1 domain to generate meizothrombin (des F1), which slowly converts to thrombin via the cleavages at Arg271 and Arg284. In the presence of thrombomodulin and phosphatidylserine/phosphatidylcholine phospholipid vesicles (PCPS), meizothrombin and meizothrombin (des F1) are better activators of protein C than thrombin (41, 42).
An additional plasma protein is factor V. Human coagulation factor V (FV) is a 330,000 MW protein, which is composed of six domains of three types in the order A1-A2-B-A3-C1-C2 (4). FV is cleaved by thrombin to remove most of the B domain and produce activated FV (FVa). Human FVa is composed of a heavy chain (A1-A2, residues 1–709) and a light chain (A3-C1-C2, residues 1546–2196), which form a non-covalent complex (5). FVa is the nonenzymatic cofactor for factor Xa (FXa) in the prothrombinase complex, which converts prothrombin to thrombin, in the presence of negatively charged phospholipids (6). Inactivation of FVa is a complex process involving APC (activated Protein C) cleavages of FVa at Arg506, Arg306 and Arg679. Cleavage at Arg506 is faster than cleavage at Arg306, and it only partially inactivates FVa while cleavage at Arg306 completely inactivates FVa and causes dissociation of the A2 domain fragments (7–10). Fully inactive FVa loses the ability to bind to FXa (11).
Still another plasma protein is factor XII. Human FXII is a single-chain protein with a MW of 76,000 and 596 amino acids. It contains, in order from N-terminus to C-terminus fibronectin type II domain, EGF domain, fibronectin type I domain, EGF domain, Kringle domain, trypsin-like serine protease domain. At least two forms of activated factor XII (FXIIa) exist. αFXIIa is formed by cleavage of the bond following Arg353, generating a two chain molecule comprised of a heavy chain (353 residues) and a light chain (243 residues) held together by a disulfide bond. Further cleavage results in FXIIa (FXIIa fragment). This is the result of cleavage at Arg334 and Arg343, resulting in two polypeptide chains (9 and 243 residues) held together by a disulfide bond (43, 44). The bulk of the N-terminal heavy chain fragment is no longer associated. Negative surface/membrane binding is mediated through this heavy chain so the resulting FXIIa fragment no longer binds to surfaces but it is still catalytically active.
The protein HGFA (hepatocyte growth factor activator) has the same domain structure as FXII (45) and is also activated by proteolytic cleavage, in this case, only one cleavage by thrombin at Arg407 (46), homologous to Arg353 in FXII. But further cleavage by kallikrein at Arg372 also results in release of the N-terminal heavy chain, which, as in FXII, is involved in surface binding (47). As known in the art, HGFA activates hepatocyte growth factor (HGF) within injured tissues where HGF plays roles in tissue repair via a mitogenic activity towards a variety of cell types.
Another FXII-like polypeptide is known by two names: PHBP (plasma hyaluronin binding protein) (48) and FVII activating protease (49). PHBP is a serine protease and is homologous to HGFA though the domain structure is not exactly the same (49, 50). This protein activates FVII, uPA, and tPA in experimental systems, but the physiological role has not been established (49, 50).